Field of the Invention
The present invention relates to a process for forming microcapsules and to the microcapsules so produced and to the use of such microcapsules in paint compositions. More particularly, the present invention relates to microcapsules having a wall containing gelatin crosslinked with quinone, wherein the core contains a fouling reducing agent such as a tributyl tin compound.
Description of Related Art
Microencapsulation involves the application of a coating around a microscopic phase of a liquid or solid core material. The first applications of microencapsulation were in carbonless copying papers and in the controlled release of drugs. Many other applications have since been explored such as described by Bayless, R., "Microencapsulation in New Areas," Chemical Engineering News, Vol. 52, p.16, August 1974.
The use of microencapsulated compounds has many advantages over the use of unencapsulated compounds. In particular, the microencapsulation separates the core material from its environment and provides a controlled release rate. The release rate of the core material and the diffusion of the core material through the capsule wall can be controlled by varying the wall composition and/or the degree of crosslinking of the walls. Furthermore, if a material is encapsulated, its useful life may be significantly extended. Also, if a material is toxic and hence difficult to handle, encapsulation of the material may reduce the threat of acute exposure and allow for easier handling.
Since many fouling reducing agents are extremely toxic and there is a desire to have these compounds act over extended periods of time, encapsulation of such compounds is desirable. Such encapsulation should serve to control the release rate of the agent and to avoid an initial high release of agent which could be environmentally unacceptable. In particular, tributyl tin compounds, such as tributyl tin chloride (TBTCl), are extremely toxic and thus excellent candidates for encapsulation.
Work by Noren et al. ("Investigation of Microencapsulated Fungicides for Use in Exterior Trade Sales Paints," Journal of Coatings Technology, 58:724 (1986)) and Porter et al. ("Extended Control of Marine Fouling," Applied Biochemistry and Biotechnology, 9:439-445, (1984)) indicate that two different coating formulations have been tested which contain bioactive microcapsules. Noren et al. have formulated an exterior paint containing microcapsules with urea-formaldehyde treated gelatin walls surrounding fungicidal compounds. The encapsulation of the fungicides allows for control of both the release and volatility of the active ingredient.
Porter et al. describe formulations of a vinyl antifouling coating containing microcapsules having gelatin and gum arabic walls crosslinked with glutaraldehyde, wherein the core is a tributyl tin chloride antifoulant. These microcapsules degrade in an aqueous environment and hence are not very useful in aqueous environments.
Haslbeck et al. in: Proceeding of the 16th International Symposium on controlled Release of Bioactive Materials, pages 273-274 (1989), describe crosslinking a gelatin/polyphosphate or gelatin/gum arabic microcapsule with glutaraldehyde and quinone. That document does not describe a method of conveniently producing such microcapsule.
Accordingly, there has been a need to find an improved microcapsule which can be conveniently manufactured and is useful for encapsulating active compounds, such as fouling reducing agents, in particular tributyl tin compounds, wherein the microcapsules allow for excellent control of the release rate of the active core material. Furthermore, there is a need to provide an improved method of encapsulating TBTC1 which, though being highly toxic, is one of the most effective fouling reducing agents known. There is a need to provide a method of producing microcapsules which results in a microcapsule exhibiting a controllable release rate and allowing for a reduced initial TBTC1 release rate so as to continue its safe use in, for example, antifoulant coating compositions.